Heretofore various sorts of communication systems have been investigated and developed. Among them the SSC system is well known.
By this SSC system, on the transmitter side, a signal such as data, sound, etc. having a narrow band is spread in spectrum into a wide band to be transmitted by using a pseudo-noise code (PN code) and, on the receiver side, this wide band signal is spread inversely into the original narrow band by means of a correlator to reproduce the signal. Recently attention is paid to this communication system, because it has always a very high reliability from the point of view that it is strong against external interference and noise, it has a high secrecy, etc.
However the greatest problematical point in the SSC system is the correlator used on the receiver side. At present, for the wireless SSC, a correlator, which is thought to be the most simple and convenient and to have a high reliability, is a device using surface acoustic wave (hereinbelow abbreviated to SAW).
In the SAW correlator there are, in general, those of correlator type (tapped delay line type) and those of convolver type. Here, although those of correlator type has a simple construction and generally a high efficiency, the temperature coefficient of the substrate has remarkable influences thereon. On the other hand, although those of convolver type are hardly influenced by variations in the temperature, they have, in general, a low efficiency. In addition, concerning the PN code described above, the code is fixed for those of correlator type, while it can be freely changed for those of convolver type.
Consequently correlators of convolver type are more easily used, provided that the efficiency is at a practically usable level.
FIGS. 14a and 14b show an example of the construction of the prior art SSC device using an SAW convolver. In the transmitter section indicated in FIG. 14a, a carrier coming from an oscillator 2 is two-phase-modulated by a signal to be transmitted (in this case digital data) in a mixer 1 to effect at first a primary modulation with a narrow band. Thereafter the output of the mixer 1 is further two-phase-modulated e.g. in another mixer 3 by using a PN code (PN) generated by a PN code generator 4 having a band substantially wider than the information signal to be transmitted and a high bit rate to spread the spectrum and then the signal is transmitted through an antenna 7 after having passed through a band pass filter 5 and an amplifier 6.
In the receiver section indicated in FIG. 14b, the spread spectrum signal received by an antenna 7' is applied to an SAW convolver 13 after having been RF amplified by an amplifier 9 and band pass filters 8 and 10.
Further, to the SAW convolver 13 is applied a reference signal obtained by two-phase-modulating a carrier coming from an oscillator generating the same carrier frequency as the received signal inputted by the SAW convolver in a mixer 12 by means of a PN code generator 19 generating a PN code (PN), which is inverted in time with respect to the PN code on the transmitter side.
The PN code for the reference signal described above is sent from the transmitter side by means of a PN code synchronizing circuit 18 to be kept in a synchronized state with the received PN code. At this time, the output of the convolver (the frequency thereof is twice as high as the inputted carrier frequency, i.e. 2fc) is led to an envelope detector 16 after having passed through a mixer 21 through a band pass filter 14 and an amplifier 15 for the PN code synchronization described above, while keeping the synchronism with the carrier of the received signal owing to a synchronizing circuit 20 (the central frequency is 2fc). The information data can be obtained by demodulation by means of a data demodulator 22, when the PN code and the carrier are in the synchronized state.
However, in the construction of the prior art SSC device described above, since it is necessary to synchronize the PN code with the carrier, it is difficult to simplify the construction of the receiver, which causes a problem in practice.